This invention is related to an aqueous cleaning solution and method of using same to remove residues from substrates. More particularly, this invention is related to an aqueous cleaning solution and method of using same to remove uncured adhesive residues from metal substrates, preferably stencils.
Adhesives are commonly used in the electronics industry to secure surface-mounted components (e.g., resistors, capacitors, inductors, transistors, integrated circuits, chip carriers and the like) to printed circuit boards prior to soldering. Stencils, which are made of metal (typically stainless steel, brass or copper), are often used to apply the adhesive to specific areas on a printed circuit board. After the adhesive has been applied to the printed circuit board, surface-mounted components are accurately placed on the precisely stenciled spots of adhesive, and the adhesive is then cured (e.g., via heat cure).
It is important to remove excess adhesive from the surface of a stencil in order to prevent the adhesive from building up and/or smearing on the stencil. Not keeping a stencil free of adhesive can cause problems, such as, e.g., the placement of an improper amount of adhesive on the circuit board or the placement of adhesive in areas where adhesive should not be. Such problems can result in product failures or in finished assemblies which are visually unacceptable.
One cleaner which has been widely used to clean stencils is isopropyl alcohol. Unfortunately, there are both environmental and safety problems associated with the use of isopropyl alcohol as a cleaning agent. For example, isopropyl alcohol is a volatile organic compound (VOC) and a dangerous fire risk.
Other cleaning systems which have been used in cleaning stencils also have problems. For example, in addition to VOC and flammability concerns, other organic solvent and semi-aqueous cleaning systems have high biological oxygen demand (BOD) and chemical oxygen demand (COD). Many aqueous systems have high pHs in addition to VOCs and relatively high BODs and CODs. In addition, many stencil-cleaning agents of all types emit unpleasant odors which can bring about worker discomfort, e.g., headaches.
To overcome the aforementioned problems, improved aqueous-based cleaning compositions for cleaning metal surfaces have been developed which are safe for workers and the environment.
For example, commonly assigned U.S. Pat. No. 5,593,504 discloses an aqueous-based cleaning composition for cleaning solder paste from surfaces, e.g., stencils, wherein the composition contains alkaline salts and a surfactant formulation composed of at least three nonionic surfactants which have cloud points that are staggered to give optimum cleaning over a broadened temperature range. Among the surfactants mentioned in the patent as being useful in the invention therein is N-(n-alkyl)-2-pyrrolidone. The composition is used at a wash temperature of from 90.degree. F. to 145.degree. F.
Commonly assigned U.S. Pat. No. 5,688,753 discloses an aqueous-based cleaning composition for removing flux residues, as well as residues of photoresists, solder masks, adhesives, machine oils, greases, silicones, lanolin, mold release, polyglycols and plasticizers, from electronic circuit assemblies. The composition contains one or more alkaline salts, an alkali metal silicate, and a surfactant mixture which includes, inter lia, an N-alkyl pyrrolidone.
Other aqueous-based cleaning compositions which are useful for removing residual contaminants such as rosin flux, photoresist, solder masks, adhesives, machine oils, greases, silicones, lanolin, mold release, polyglycols and plasticizers, from metal substrates are disclosed, e.g., in commonly assigned U.S. Pat. Nos. 5,234,505; 5,261,967; 5,464,553; 5,433,885; 5,393,448; 5,234,506; 5,264,047; 5,431,847; and 5,397,495. The compositions disclosed in these patents contain at least one alkaline salt and preferably further contain a corrosion inhibitor, an antifoam agent, a hydrotrope, and one or more surfactants. One of the surfactants disclosed in U.S. Pat. No. 5,431,847 is N-alkyl-2-pyrrolidone. The aforementioned patents teach that the wash temperature used therein can range from room temperature to about 180.degree. F. In the examples set forth in these patents, the wash temperature used was typically about 160.degree. F.-165.degree. F.
Copending, commonly assigned U.S. application Ser. No. 08/852,065 (filed May 6, 1997), now U.S. Pat. No. 5,789,363 discloses an aqueous-based cleaning composition for removing industrial-type soils (e.g., dirt, grease, oil, ink and the like) from metal parts. The composition contains water, an alkalinity-providing agent, and a surfactant mixture composed of N-octyl-2-pyrrolidone and an aminocarboxylic acid surfactant, e.g., N-coco-.beta.-amino propionic acid. The composition can be used at a wash temperature of from about 90.degree. F. to about 180.degree. F., with a temperature of from about 120.degree. F. to about 160.degree. F. being preferred.
Although the aqueous-based cleaning compositions disclosed in the aforementioned commonly assigned patents and copending, commonly assigned application are highly effective in removing solder and oily residues from metal surfaces, they are somewhat less effective in removing uncured adhesive residues from substrates, particularly metal substrates, more particularly from stencils.
Removing uncured adhesive residues from stencils is extremely difficult for most cleaning agents. Removal of such residues is particularly difficult for aqueous-based cleaning compositions because of the organic nature of the residues. Another factor which makes removal of uncured adhesive residues from stencils particularly difficult is that only moderate wash temperatures can be used in cleaning stencils because the materials which hold the stencil in place cannot withstand temperatures above about 135.degree. F. Thus, unlike methods for removing oil from metal surfaces wherein relatively high temperatures (i.e., 150.degree. F. to 180.degree. F.) are generally required for acceptable soil removal, methods for cleaning stencils are usually restricted to temperatures of less than 135.degree. F. Therefore, an aqueous cleaning composition capable of effectively removing oily-type soils may be less effective at removing uncured adhesive residues, particularly at relatively low wash temperatures in the case of stencil substrates.
Accordingly, a primary object of this invention is to provide an aqueous cleaning composition which has improved ability to remove uncured adhesive residues from substrates.
Another object of this invention is to provide an aqueous cleaning composition which has improved ability to remove uncured adhesive residues from metal substrates.
A further object of this invention is to provide an aqueous cleaning composition which has improved ability to remove uncured adhesive residues from stencils.
Yet another object of this invention is to provide an aqueous cleaning composition which has improved ability to remove uncured adhesive residues at relatively low wash temperatures, e.g., less than about 135.degree. F.
A still further object of this invention is to provide a method of removing uncured adhesive residues from a substrate using an aqueous cleaning composition having the properties set forth in the preceding objects.
These and other objects which are achieved according to the present invention can be discerned from the following description.